1. Field of the Invention
The present invention relates to a backlight device particularly suitable for being applied to a transmissive liquid crystal display apparatus, and to a liquid crystal display apparatus.
2. Description of the Related Art
Recently, a considerably thin display device such as an LCD (Liquid Crystal Display), PDP (Plasma Display Panel) and the like is proposed to be commercially available as the display device replacing a CRT (Cathode Ray Tube) for a television receiver. Particularly, since an LCD apparatus using an LCD panel is capable of being driven by low power consumption and a large-sized LCD panel has become capable of being manufactured at low costs, the LCD apparatus using an LCD panel is widely used and the technical research and development is advanced.
The above LCD apparatus mainly uses a backlight system in which a color image is displayed by a backlight device that illuminates from the rear side the whole rear surface of the transmissive LCD panel including a color filter.
A CCFL (Cold Cathode Fluorescent Lamp) which emits white light by using a fluorescent tube and an LED (Light Emitting Diode) are considered to be promising as a light source of the backlight device (refer to Patent Reference 1, for example).
Particularly, with development of a blue LED, respective LEDs of emitting red light, green light and blue light that are three primary colors in light are prepared, and white light with high color purity can be obtained by mixing red light, green light and blue light emitted therefrom. Therefore, with those LEDs being a light source of the backlight device, color purity through an LCD panel becomes high and so color reproduction range can be greatly expanded in comparison with the range of a CCFL. Further, luminance of the backlight device can greatly be improved by using an LED chip of high power.
As described above, in the case where an LED is used as a light source of a backlight device, when the LED is arranged directly under a light-emitting surface, namely, when a direct type LED is employed, uneven luminance and uneven chrominance may be caused, because the LED is a point source of light.
As a method to solve the above problem, it is conceivable to enlarge the distance between the LED and a diffusion plate. However, if the distance is enlarged, the thickness of the backlight may be extremely large in comparison with the backlight using a CCFL. It is desirable for an LCD apparatus to be further lightweight and shaped thin, and so it is also desirable for the backlight to be thinner.
Then, as a method to reduce the thickness, a structure is proposed in which a light-guiding plate is used to reflect light emitted from a light source arranged at a part of the plate multiple times so that a surface light source is obtained.
In the structure, a critical angle formed by the difference in refractive index between the light-guiding plate and air around the plate is used to confine light with an incident angle larger than the critical angle, thereby making light uniform. Typically, a reflective film or diffuse reflective sheet is formed covering the surface of the light-guiding plate to improve efficiency in using light.
[Patent Reference 1] Japanese Published Patent Application No. H8-136917
[Patent Reference 2] Japanese Published Patent Application No. 2004-265646